The US Supreme Court is to review the legality of lethal injections, the means of execution in most US states.

The high court has agreed to hear challenges from two death row inmates Ralph Baze and Thomas Clyde Bowling Jr.

The pair sued the state of Kentucky in 2004, saying lethal injections were cruel and a violation of civil rights.

The case could affect the way inmates are executed in the US. Lawyers for the men said the court had not reviewed the issue for more than 100 years.

"This is probably one of the most important cases in decades as it relates to the death penalty," said legal representative David Barron.

'Cruel punishment'

Lethal injection is used in all the 38 states that have capital punishment except Nebraska, which requires electrocution.

LETHAL INJECTIONS

Sodium pentothal - anaesthetic

Pancuronium bromide - paralyses entire muscle system

Potassium chloride - stops the heart

The standard method is a combination of three chemicals - one which makes the inmate unconscious, another that paralyses all muscles except the heart, and a final drug which stops the heart, causing death.

Some argue that the executions are often slow and painful, so violating a constitutional ban on "cruel and unusual punishment".

Last year, the Supreme Court ruled that death row inmates could file last-ditch challenges to lethal injections.

The unanimous ruling came in the case of Clarence Hill, who argued that the chemicals used would cause unnecessary pain and violate his civil rights.

The ruling only stated that Hill could bring the challenge and did not address whether the method of lethal injection violated the US constitution.

However Hill's challenge did not succeed and he was executed in September 2006.